Earth’s Rotationhttp://81science.com/81Science.com/Earths_Rotation.html

Bell Work

Earth’s Rotation – Causes daily changes

• The spinning of earth on its AXIS – the imaginary line through the planet from the North Pole to the South Pole – is its rotation

• The axis of the earth is tilted 23.5° from a line perpendicular to the plane of its orbit of the sun.

Earth’s Rotation

• As the earth revolves around the sun, Earth’s axis remains tilted at 23.5°

• The north end of the axis points toward the north star (Polaris)

• Earth rotates 360° from west to east (counter clockwise looking down from the north pole) in 24 hours, at an angular rate of 15° per hour (360°/24 hours = 15°/hour)

Tilt of the axis always stays the same

1. Evidence of Earth’s Rotation

• 1851, Foucault’s pendulum will appear to change in a predictable way.

• Why is this evidence? There is no force acting on the pendulum to make its plane of oscillation rotate or go around its own axis

• It would continue to swing in the original path if Earth did not rotate.

No force that makes pendulum rotate – must be earth that is rotating!!!

• http://video.search.yahoo.com/search/video;_ylt=A0WTb_uxJKFMIi0A0zGJzbkF?ei=UTF-8&p=focault%20pendulum&fr2=tab-img&fr=yfp-t-944

2. Evidence for Rotation

• Coriolis Effect – the tendency of objects moving over the earth (air, water, planes, projectiles) to be deflected (curve away) from a straight line path. The French scientist, Gaspard Coriolis, first explained the deflection of objects moving over the surface due to Earth’s rotation

• The deflection is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

• This deflection occurs because Earth’s surface is rotating with respect to the objects.

• http://video.search.yahoo.com/video/play?n=21&b=121&ei=utf-8&fr=yfp-t-944&fr2=sp-qrw-corr-top&tnr=20&p=coriolis+effect&vid=249752912042&dt=1238396400&l=304&turl=http%3A%2F%2Fts3.mm.bing.net%2Fvideos%2Fthumbnail.aspx%3Fq%3D249752912042%26id%3D8db081bc51d94d2c3b2a7f1cb400cee5%26bid%3DCQmGKvo0IYTTDA%26bn%3DThumb%26index%3Dch1%26url%3Dhttp%253a%252f%252fwww.youtube.com%252fwatch%253fv%253d5dZrfKrJ4dc&rurl=http%3A%2F%2Fwww.youtube.com%2Fwatch%3Fv%3D5dZrfKrJ4dc&tit=The+Coriolis+Effect&sigr=11a4r72k2&newfp=1

What are the APPARENT MOTIONS of Celestial Objects

• An apparent motion is a motion an object appears to make

• An apparent motion can be real or an illusion

• For example, the stars appear to move across the sky from east to west

• However, the apparent motion is caused by Earth’s rotation

Stars in constellations are often very far from one another but we see them in

the same direction in the sky.

Big Dipper

Pointer Stars point to Polaris

Imagine all the stars moved down onto a big glass sphere

around the Earth.

Stars

Celestialequator

Stars

celestial sphere is stationary

Earth Rotates Counterclockwise

Stars

•Looking upward we see the stars fixed on the celestial sphere

•Earth’s eastward rotation makes the stars between the equator and north celestial pole appear to move westward

•Rise in the east and set in the west.

• Timed photography showing stars rising in the east

Stars appear to be moving from east to west. Stars rising in the

east.

Stars appear to be moving from east to west.

Rise in the east and set in the west.

Northern part of the sky around Polaris

CCW

Moving west to east

Earth’s counter clockwise rotation makes the stars appear to rotate

counter clockwise around the north celestial pole (Polaris).

The complete circular path can be seen for stars in the northern part

of the sky around Polaris

Circumpolar constellations never set.Circumpolar constellations change with latitude… sky changes with latitude

Daily Motions

• Some of the stars near Polaris appear to move in a complete circle in 24 hours.

• The paths of all celestial objects moving in the sky are circular or parts of a circle called an arc.

• All motion occurs at a constant rate of approximately 15° per hour or 360° in 24 hours.